For the past quarter of a century, crosslinked homopolymers of acrylic acid have provided a route to permanent suspension of insoluble or incompatible ingredients in water-based systems. Many modern emulsion products rely on such polymers of acrylic acid to provide excellent stability against creaming or coalescence during storage and to allow easy flow of the product from its container during application.
The homopolymers of acrylic acid should not be regarded solely as a means of increasing viscosity of a formulation. An increase in viscosity alone is not sufficient to provide permanent suspension of a dispersed phase. Stokes' Law tells us that merely increasing viscosity will delay but not stop separation or sedimentation of particles or droplets suspended in a liquid. This assumes, of course, that the particles are too large to be suspended by Brownian motion.
Systems thickened with homopolymers of acrylic acid display plastic rheology. Materials that exhibit plastic flow characteristics will flow only after the applied shearing stress exceeds a critical minimum value. This minimum shearing stress is designated as yield value. At stresses below the yield value, the system displays the rheology of a solid whereas at shearing stresses above the yield value, the system exhibits liquid-like rheology. This explains the ability of the polymers of acrylic acid to suspend insoluble particles in systems at rest but to flow easily once the yield value is exceeded.
Although homopolymers of acrylic acid have been used for many years to suspend insoluble or incompatible ingredients in water-based systems, such polymers are not compatible with anionic surfactants and do not effectively suspend certain ingredients in the manufacture of personal care products.